In recent years, solid-state imaging devices have been developed to have increased number of pixels up to five million pixels or the like, enabling to capture still pictures like silver salt pictures and to capture moving pictures. When capturing moving pictures, several hundred thousand pixels are used for capturing, so in order to effectively utilize the extra photoelectric conversion elements, a means for mixing signals of respective photoelectric conversion elements of pixels is adopted generally (for example, refer to Patent Reference 1).
FIG. 1 is a diagram showing the configuration of a signal readout circuit of a conventional solid-state imaging device.
As shown in FIG. 1, a signal readout circuit includes memories (hereinafter, referred to also as capacitors or accumulation circuits) 101, 102, 103 and 104 for accumulating signals from the respective pixels of the solid-state imaging device respectively, and MOS transistors 105, 106, 107 and 108, and the like.
The MOS transistors 105, 106, 107 and 108 are conducted when a high-level signal is applied to the respective gates of these MOS transistors, thereby causing the memories 101, 102, 103 and 104 to be in a state of being connected in parallel, and an average of the signals from the pixels accumulated in the memories 101, 102, 103 and 104 is outputted to a signal output line 109.    Patent Reference 1: Japanese Patent Application Laid-Open No. 2001-292453 (pp. 1-11, FIG. 4)